Method for preparing dialkyl carbonate

ABSTRACT

The present invention provides a method for preparing dialkyl carbonate from urea or alkyl carbamate and alkyl alcohol using an ionic liquid comprising a cation, which produces a hydrogen ion, and a hydrophobic anion containing fluorine with high temperature stability in the presence of catalyst containing a metal oxide or hydrotalcite. Since the present invention can prepare dialkyl carbonate at a pressure lower than those of existing methods, it does not require an expensive pressure control device and peripheral devices for maintaining high pressure including the installation cost. It is also the method for preparing a dialkyl carbonate with high yield, thus improving economical efficiency. Moreover, the method of the present invention hardly produces any waste during the process and is thus an eco-friendly method.

TECHNICAL FIELD

The present invention relates to a method for preparing dialkylcarbonate from urea or alkyl carbamate and alkyl alcohol using a metaloxide catalyst and an ionic liquid comprising a cation, which produces ahydrogen ion, and a hydrophobic anion containing fluorine with hightemperature stability.

BACKGROUND ART

Dimethyl carbonate (hereinafter ‘DMC’), a typical dialkyl carbonate, iscolorless, odorless and has an environment-friendly molecular structurewithout any known toxicity to human body. Since DMC contains highlyreactive groups of methoxy group, carbonyl group and carbonyl methyloxygen group in its molecule structure, it can be used to replace highlytoxic phosgene as carbonylating agent and also dimethyl sulfate andmethyl halides as methylating agent.

DMC has an excellent solubility and is thus used as anenvironment-friendly solvent to replace halogenated solvents such aschlorobenzene. It has been widely used as a substitute for phosgene as araw material for polycarbonate, an additive for improving the octanenumber of automotive fuel, and an electrolyte for rechargeablebatteries.

DMC has been mainly synthesized by methanol and phosgene in the presenceof highly concentrated sodium hydroxide solution. High toxicity ofphosgene and corrosiveness of chlorine ion have been limiting alarge-scale production of DMC and its applications.

In 1983, Enichem Company in Italy developed a non-phosgene method tosynthesize DMC by oxidative carbonylation of methanol and carbonmonoxide with oxygen in the presence of a monovalent copper chloridecatalyst. However, this method has some problems such as use of a toxiccarbon monoxide as a raw material, a low conversion rate and a highenergy cost due to unreacted methanol and by-product water. Further,because the copper chloride (I) catalyst is readily oxidized to adivalent copper ion, its catalytic activity is reduced. Further, it alsorequires continued monitoring of the reaction chamber against thecorrosion and explosion. In addition, due to the presence of a smallamount of chloride ions in the product, the refining cost isconsiderably increased when DMC is used as an electrolytic solution in asecondary lithium battery.

Another conventional method for preparing DMC is Ube process. Theprocess proceeds in two steps in gas-phase: in the first step, methanolreacts with nitrogen oxide (NO) and oxygen to give methylnitrite (MN)and water, without any catalyst. In the second step, MN reacts withcarbon monoxide to produce DMC, in the presence of a palladium supportedcatalyst. In the catalytic process, the NO produced in the latterreaction is converted again to MN. Although the cost of energy for theseparation and purification process is relatively low in this process,the use of the highly toxic and corrosive carbon monoxide and NOrequires an anti-corrosion reaction chamber, an anti-explosion safetydevice for a precise controlling of raw materials concentration. Also,there is a problem that the reactants may leak.

Still another conventional method for preparing DMC is Texaco process inwhich ethylene oxide (or propylene oxide) and carbon dioxide are reactedwith each other at high pressure in the presence of a catalyst to formethylene carbonate (or propylene carbonate) and thus prepared DMC andethylene glycol (propylene glycol) through ester interchange reactionwith methanol Unlike the above-mentioned two conventional processes, theTexaco process does not use carbon monoxide and is thus considered avery safe process. However, since the process is performed at hightemperature and pressure, there is still a possibility of explosion dueto leakage of ethylene oxide. Moreover, the conversion rate is not veryhigh, and thus it still requires a large amount of energy for theseparation and purification of DMC and ethylene glycol as products fromunreacted materials.

Yet still another method for preparing DMC is a method for directlysynthesizing carbon dioxide and methanol at high temperature andpressure in the presence of a catalyst. However, the yield of DMC isextremely low in a thermodynamic equilibrium state

Recently, a method for preparing dialkyl carbonate by directlysynthesizing urea and methanol in the presence of a catalyst has beenactively studied. This method has the advantages that inexpensive ureais used as a raw material and, since water is not produced as aby-product, a ternary azeotropic mixture such as methanol-water-DMC isnot formed, thus simplifying the separation and purification process.Moreover, the ammonia produced as a by-product can be reused by a ureaformation by synthesizing ammonia with carbon dioxide, and thus it ispossible to provide an environment-friendly process which does notproduce by-products.

The methods for preparing DMC from urea and methanol are as follows.Method (1) for synthesizing DMC from urea and methanol in the presenceof a zinc acetate catalyst (S. Bowden., E. Buther, J. Chem. Soc. 1939,vol. 78) and method (2) for synthesizing various dialkyl carbonates fromurea and primary aliphatic alcohol in the presence of an organic metalcompound catalyst such as magnesium methoxide [Mg(OCH₃)₂] and an organicphosphine catalyst such as triphenylphosphine (PPh₃) (Peter Ball, HeinzFullmann, and Walter Heintz, “Carbonates and Polycarbonates from Ureaand Alcohol”, Angrew. Chem. Int. Ed. Engl. 1980, vol. 19, No. 9, pp718-720, WO 95/17369). However, the above conventional methods (1) and(2) for preparing DMC have the problem that the yield is low.

Another a method (3) for preparing DMC in the presence of a catalystcomplex comprising an organotin compound and a high boiling electrondonor compound containing polyglycol ether such as triethylene glycoldimethyl ether (PGDE) is disclosed in U.S. Pat. No. 6,010,976 by J YongRye, and various process patents are disclosed in U.S. Pat. No.6,392,078 B1 and U.S. Pat. No. 7,314,947 B2 based on method (3).However, the disclosed catalyst complex has the disadvantages that thecatalytic activity is rapidly reduced by water contained in a rawmaterial as an impurity and it has toxicity to the ecosystem. Moreover,the high boiling oxygen containing polyglycol ether compound used as aco-catalyst is decomposed or polymerized at high temperature, and thusthe activity of the co-catalyst is reduced due to the change inviscosity by thermal decomposition. Further, the organotin catalyst andthe polyglycol ether compound used as co-catalyst are to be discardeddue to the difficulty in their recycling, and this raises anenvironmental issue.

There is another method (4) for preparing DMC by directly reacting ureawith methanol in a catalyst rectification reactor or distillation columnusing alumina and silica supports on which metal oxides such as Zn, Pb,Mn, La, and Ce and alkali oxides such as K, Na, Cs, Li, Ca, and Mg areimpregnated as reaction catalysts (U.S. Pat. No. 7,271,120). This methodis an improved method that can easily separate a catalyst from a givenproduct. However, because the reaction temperature is much higher thanthe boiling point of methanol, it is necessary to maintain thevapor-liquid equilibrium at high pressure. Moreover, if the producedammonia and DMC) are not discharged, the reaction yield is reduced, andthe amount of by-products such as N-methyl carbamate (N-MC) andN,N-dimethyl carbamate (NN-DMC) is increased due to a side reactionbetween methyl carbamate (MC) as an intermediate product and DMC.Therefore, in order to improve the reaction yield and distillationefficiency of DMC at the reaction temperature higher than the boilingpoint of methanol and under the high vapor pressure of methanol duringthe preparation of DMC by the reactive distillation. It is necessary tomaintain the reaction temperature and the pressure at the vapor-liquidequilibrium and it is further necessary to discharge ammonia anddistillate to obtain DMC.

Here, the distillate is obtained as an azeotropic mixture of DMC andmethanol, and the concentration of DMC in the azeotropic mixture isreduced at high pressure, which reduces the productivity. Although theamount of by-products in method (4) is smaller than that of method (3),the amount of by-products such as N-MC and NN-DMC is increased due to ahigh reactivity of the synthesized DMC which is reacted with MC as anintermediate product at high pressure as represented by the followingreaction scheme, which is well-known in the art (Yoshio Ono, “Dimethylcarbonate for environmentally benign reaction”, Pure & Appl. Chem.,1996, Vol. 68, No. 2, pp 367-375).

In addition, the method (5) for preparing DMC using polyethylene glycoldimethyl ether (PGDE, MW 250 to 270) as an organic solvent, which isstable at atmospheric pressure and reaction temperature, whileinhibiting the decomposition of urea and MC in the presence of variousmetal catalysts is disclosed (Bolin Yang et al. “Synthesis of dimethylcarbonate from urea and methanol catalyzed by the metallic compounds atatmospheric pressure”, Catalysis communications, 2006, vol. 7, p.472-477). PGDE used is an organic solvent used as a medium formaintaining the reaction temperature at atmospheric pressure and as anelectron donor or used to inhibit the decomposition of raw materials.However, this method has also some problems such as recycling of usedPGDE and catalysts due to decomposition, consumption during reaction andlow yield per unit time.

Moreover, a method (6) for preparing dialkyl carbonate by the reactionof urea or alkyl carbamate and alkyl alcohol using a quaternary ammoniumionic liquid such as tetramethylammonium hydrogencarbonate methyl esterand tetramethylammonium carbamate and an organotin catalyst at atemperature of 160° C. and a pressure of 20 atmospheres is disclosed inU.S. Pat. No. 5,534,649. However, method (6) for preparing DMC usingmethyl carbamate, methanol, and an ionic liquid has the problem that themaximum yield of DMC is very low (4.13%).

In general, the reaction process of synthesizing dialkyl carbonate bythe reaction of alkyl alcohol and urea can be represented by thefollowing reaction scheme 1:

It can be seen from the above reaction scheme 1 that when the produceddialkyl carbonate and ammonia are effectively discharged from thereactor, the equilibrium reaction will shift to the forward direction,thereby increasing the reaction rate and yield. In the synthesis of DMC,due to the low boiling point of methyl alcohol as a reactant, it isnecessary to increase the reaction pressure (15 to 25 atmospheres) inorder to maintain the reaction temperature, and thus the solubility ofdialkyl carbonate and ammonia produced at high pressure is increased. Asa result, the equilibrium constant becomes low, thereby reducing thereaction rate and yield. Moreover, since the solubility of DMC producedat high pressure is also increased, the amount of undesired by-productssuch as N-MC and NN-DMC is increased.

DISCLOSURE OF INVENTION Technical Problem

Technical problems on conventional methods are limitations on alarge-scale production and applications due to high toxicity ofphosgene, carbon monoxide and corrosiveness of chlorine ion, lowconversion rate and a high energy cost due to unreacted methanol andby-product water.

Although the cost of energy for the separation and purification isrelatively low in some processes, there are still some disadvantagessuch as the use of the highly toxic and corrosive carbon monoxide and NOrequires an anti-corrosion reaction chamber, an anti-explosion safetydevice for preventing reactants leakage.

A direct synthesizing method of DMC using carbon dioxide and methanolalso has some disadvantages such as the low yield of DMC due to athermodynamic equilibrium state at high temperature and pressure.

In addition, the method for preparing DMC using an organotin catalystand polyethylene glycol dimethyl ether (PGDE, MW 250 to 270) as an hightemperature organic solvent as co-catalyst, which has the disadvantagesthat the catalytic activity is rapidly reduced by water and hightoxicity. Moreover, the high boiling oxygen containing polyglycol etheris decomposed or polymerized at high temperature, and thus the activityof the co-catalyst is reduced due to the change in viscosity by thermaldecomposition.

Solution to Problem

To accomplish the above objects of the present invention, there isprovided a method for preparing dialkyl carbonate from urea or alkylcarbamate and alkyl alcohol using an ionic liquid comprising a cation,which produces a hydrogen ion (H⁺), and a hydrophobic anion containingfluorine and a catalyst containing at least one selected from the groupconsisting of an alkali earth metal oxide, a transition metal oxide, arare earth oxide, and a hydrotalcite. Therefore, an object of thepresent invention is to provide a new method for preparing dialkylcarbonate from urea or alkyl carbamate and alkyl alcohol using an ionicliquid at low pressure.

Advantageous Effects of Invention

Since the ionic liquid used in the present invention has excellentstability by contact with water and air, it does not flow out with thereaction products during the reaction. Moreover, since the ionic liquidcan easily dissolve urea as a raw material and alkyl carbamate as anintermediate product, it is possible to maintain a high concentration ofraw material without decomposition and sublimation at high temperature,thereby increasing the reaction rate. Also, it is possible to preventthe generation of by-products and increase the productivity due to theincrease of activity of the metal oxide catalyst even at hightemperature and low pressure. Therefore, the present invention is anenvironment-friendly preparation method that significantly reduces theamount of energy used and produces no waste. Especially by adapting lowpressure distillation, it will increase the concentration of the productat the azeotropic point. The amount of alkyl alcohol as a circulatingraw material is also reduced, which reduces the size of the apparatuswhile improving its productivity. Further, the present invention enablesto increase the reuse of the ionic liquid and catalyst including thehigh yield of dialkyl carbonate.

Mode for the Invention

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

A method for preparing dialkyl carbonate by reacting alkyl alcohol withurea or alkyl carbamate in the presence of:

an ionic liquid comprising a cation capable of generating a hydrogen ion(H±), and a hydrophobic anion containing fluorine, and a catalystcomprising at least one selected from the group consisting of an alkaliearth metal oxide, a transition metal oxide, a rare earth oxide, and ahydrotalcite.

In particular, the present invention aims at preparing dialkyl carbonateby reacting urea and alkyl alcohol under atmospheric pressure and at atemperature of 140 to 240° C., preferably 150 to 220° C. If the reactiontemperature is below 140° C., the reaction rate is significantlyreduced, whereas if it exceeds 240° C., the amount of by-products issignificantly increased. Thus, it is preferable that the reaction isperformed in the above temperature range.

Moreover, the method for preparing dialkyl carbonate does not requirehigh pressure corresponding to the vapor pressure of alkyl alcohol (15to 25 atm in the case of methanol) depend on the reaction temperaturerequired in the existing preparation methods. The method formanufacturing dialkyl carbonate of the present invention can be carriedout at low pressure as well as at high pressure, preferably at apressure of 0.1 to 1.5 atm, more preferably at atmospheric pressure,through product recycling process including methyl alcohol. However,unlike the conventional methods, the method according to the presentinvention enables to manufacture dialkyl carbonate at any pressureregardless of the vapor pressure of alkyl alcohol and thus the presentinvention is not limited thereto.

The ionic liquid used in the present invention is in the form of[cation][anion] pairs and serves as a solvent and a heat medium capableof maintaining urea and methyl carbamate as an intermediate product in aliquid form at high temperature and low pressure. Moreover, the ionicliquid produces hydrogen ions to improve the reaction rate of thecatalyst. Further, since the ionic liquid easily dissolves urea as areactant, it is possible to increase the concentration of urea. Inaddition, the ionic liquid is not decomposed at the reaction temperatureand has no reactivity with alkyl alcohol, ammonia, and water as areactant.

The [cation] is a cation which produces a hydrogen ion (H⁺), preferablya cation which produces a hydrogen ion (H⁺) and contains at least onesubstituent selected from the group consisting of a C₁-C₁₆ hydroxyalkylgroup, a C₁-C₁₆ alkoxy group, and a C₁-C₁₆ alkyl group. TheN-substituent s backbone may comprise a quaternary ammonium cation, animidazolium cation, a pyridium cation, a pyrazolium cation, apyrrolinium cation, a quaternary phosphonium cation, a thiazoliumcation, or a sulfonium cation, preferably a quaternary ammonium cation,an imidazolium cation, a pyridium cation, or a pyrazolium cation. Here,if the carbon number of the substituent exceeds 16, the ionic liquid mayhave a melting point higher than the reaction temperature regardless ofthe anion. Thus, it is preferable that the substituent has the abovenumber of carbon atoms. Moreover, it is preferable to use an ionicliquid which hash has a melting point lower than the reactiontemperature, and more preferably an ionic liquid which is a liquid atroom temperature (i.e., a room temperature ionic liquid). However, thecarbon number is not particularly limited in the present invention.

The [cation] will be described in more detail below. The quaternaryammonium cation may have at least one substituent selected from thegroup consisting of a C₁-C₁₆ hydroxyalkyl group, a C₁-C₁₆ alkoxy group,and a C₁-C₁₆ alkyl group. Preferably, the quaternary ammonium cation mayhave at least one substituent selected from the group consisting of aC₁-C₅ hydroxyalkyl group, a C₁-C₅ alkoxy group, and a C₁-C₅ alkyl group.More preferably, the quaternary ammonium cation may be ahydroxymethyltrimethylammonium cation (CH₃)₃N⁺CH₂OH), ahydroxyethyltrimethylammonium cation (CH₃)₃N⁺C₂H₄OH, choline), ahydroxyethyltriethylammonium cation [(CH₂H₅)₃N⁺C₂H₄OH], ahydroxyethyltripropylammonium cation [(C₃H₇)₃N⁺C₂H₄OH], ahydroxyethyltributylammonium cation [(C₄H₉)₃N⁺C₂H₄OH], atetraethylammonium cation [(C₂H₅)₄N⁺], or a tetrabutylammoniumcation[(C₄H₉)₄N⁺]. The imidazolium cation may have at least onesubstituent selected from the group consisting of a C₁-C₁₆ hydroxyalkylgroup, a C₁-C₁₆ alkoxy group, and a C₁-C₁₆ alkyl group. More preferably,the imidazolium cation may be a 1,3-di(C₁-C₅)alkyl-imidazolium cation ora 1-hydroxy(C₁-C₅)alkyl-3-(C₁-C₅)alkyl imidazolium cation. Moreover, theN-hydroxyalkylpyridium cation may be an N-hydroxy(C₁-C₁₆)alkylpyridiniumcation, preferably an N-hydroxy(C₁-C₃)alkylpyridinium cation. Thepyrazolium cation may be a1-hydroxy(C₁-C₁₆)alkyl-2-(C₁-C₁₆)alkylpyrazolium cation, preferably a1-hydroxy(C₁-C₅)alkyl-2-(C₁-C₅)alkylpyrazolium cation.

The [anion] of the ionic liquid may be a compound containing fluorinefor the good stability at high temperature and in water. The anion maybe bis(trifluoromethylsulfonyl)imide (NTf₂), trifluoromethanesulfonate(OTf), tris(trifluoromethylsulfonyl)methanide, (CTf₃). However, in thecase where a halogen anion (F⁻, Cl⁻, Br⁻, or I⁻) is used as the anion,the melting point of the ionic liquid is too high, the thermal stabilityis low, and the solubility to water is increased. As a result, it isdifficult to recover the ionic liquid owing to an easy decomposed anionunder the reaction temperature and reacted with a metal oxide catalystto inhibit the activity of the catalyst. Moreover, in the case oftetrafluoroborate (BR) or hexafluorophosphate (PF₆), it reacts withalkyl alcohol as a reactant to produce by-products such astrialkoxyboroxane and alkoxy phosphorus compounds or generatehydrofluoric acid (HF), which causes corrosion to a reactor.

In the present invention, the catalyst may be at least one selected fromthe group consisting of an alkali earth metal oxide, a transition metaloxide, a rare earth oxide. Moreover, the catalyst may be impregnatedonto a support such as silica, alumina, titania, zirconia or ceria.Further, the catalyst may be a mixed oxide with a crystalline compositeoxide such as hydrotalcite. Examples of the alkali earth metal oxide,the transition metal oxide, the rear earth oxide include CaO, MgO, ZnO,CuO, PbO, La₂O₃, Y₂O₃ which can be impregnated onto a support. Althoughthe size of the catalyst is not particularly limited, the reaction rateis increased and the yield is improved when a nano-sized catalyst isused. The amount of catalyst used is preferably 1 to 10 parts by weightper 100 parts by weight of the ionic liquid. If the amount of catalystused is less than 1 part by weight, the reaction rate and DMC yield aredecreased, respectively. Meanwhile, if it exceeds 10 parts by weight,the yield improvement effect by an increase in the amount of catalystused is reduced due to an increase in viscosity, which is uneconomic.

The hydrotalcite may be a mixed oxide prepared by drying or calciningpowder having a structure of Mg_(x)Al_(y)(OH)_(2(x+y))(CO₃)_(y/2).mH₂O.In the structure of the hydrotalcite, the ratio of x/y may be in therange of 3 to 9, and m represents the number of water for crystals. Itis preferable that the hydrotalcite is completely dried at a temperatureabove 120° C. to prevent the decomposition of urea into ammonia andcarbon dioxide by the reaction with water in the hydrotalcite. It ismore preferable that the hydrotalcite is calcined at a temperature above400° C. to be a mixed oxide of magnesium and alumina to prevent thedecomposition of urea, thus increasing the yield.

The alkyl alcohol as a main raw material for the preparation of dialkylcarbonate may be a linear, branched, or cyclic alkyl alcohol,preferably, but not particularly limited to, C₁-C₆ alkyl alcohol interms of reactivity; however, the present invention is not limitedthereto. The alkyl alcohol may be fed through a conduit at a constantflow rate of 0.1 to 10 ml/min relative to 100 parts by weight of theionic liquid, preferably at a constant flow rate of 0.1 to 5 ml/min. Ifthe flow rate of the alkyl alcohol is less than 0.1 ml/min, theproductivity is reduced, whereas if it exceeds 10 ml/min, it isdifficult to control the reaction temperature due to the heat ofvaporization of alkyl alcohol. Moreover, the increase in the amount ofunreacted urea and alkyl alcohol leads to generation of a condensatewith a low-concentrated dialkyl carbonate, and increases the cost forpurifying dialkyl alcohol, thus being commercially not advantageous. Itis still possible to increase the reaction yield by controlling the formof the reactor, the disperser of alkyl alcohol, and the stirring rate toincrease the flow rate of alkyl alcohol, and thus the present inventionis not limited to the flow rate.

The amount of urea or alkyl carbamate as a main raw material for thepreparation of dialkyl carbonate may be 1 to 30 parts by weight per 100parts by weight of the ionic liquid, preferably 1 to 25 parts by weight.Moreover, the molar ratio of alkyl alcohol to urea or alkyl carbamatemay be 5 to 30:1. If the molar ratio of alkyl alcohol to urea or alkylcarbamate is less than 5, the yield of dialkyl carbonate is reduced.Meanwhile, if it exceeds 30, a large amount of unreacted urea and methylcarbamate is distilled to reduce the selectivity, and the amount ofalkyl alcohol recycled is increased, thus increasing the energy requiredfor the recycling of alkyl alcohol. Therefore, the molar ratio of alkylalcohol to urea or alkyl carbamate may be maintained within the aboverange, preferably 5 to 25:1, more preferably 5 to 20:1. Although theflow rate of alkyl alcohol to the reactor may differ according to thereactor and disperser type and the stirring rate, the alkyl alcohol isfed in 100 parts by weight of the ionic liquid at a flow rate of 0.1 to10 ml/min, preferably at a flow rate of 0.25 to 5 ml/min, morepreferably at a flow rate of 0.25 to 3 ml/min. If the flow rate of alkylalcohol is less than 0.1 ml/min, the selectivity becomes good, but thereaction rate is reduced. Meanwhile, if the flow rate exceeds 5 ml/min,the selectivity is reduced, but the reaction rate is increased. However,the excessive feeding of alkyl alcohol may cause a problem in thecontrol of the reaction temperature, and the energy required for theseparation and purification is increased.

The alkyl of the alkyl carbamate may have a carbon number of 1 to 6,preferably 1 to 3, which is advantageous since there is no sterichindrance during reaction with alkyl alcohol can be used; however, thepresent invention is not limited thereto.

The ionic liquid and catalyst used in the present invention may becollected and recycled. The ionic liquid may be collected when theactivity of the catalyst decreases or when the catalyst is contaminated.The catalyst as a form solid can be recovered using centrifugation orfiltration.

The used ionic liquid contaminated with the carbonized organic materialsmay be decolorized by using activated carbon. Inorganic impurities suchas metal oxides dissolved in the ionic liquid may be purified by astrong acid such that a metal salt solution of the upper layer isdiscarded. Thus obtained lower layer ionic liquid washed with somedistilled water and further washed with ether if necessary. The purifiedionic liquid is dried by the vacuum-rotary-evaporator and then reused.Therefore, the method for preparing dialkyl carbonate of the presentinvention is an environment-friendly method which does not dischargeother waste.

Next, the present invention will be described in more detail withreference to Examples, but the scope of the present invention is notlimited to the following Examples.

EXAMPLES Synthesis Example Synthesis of Ionic Liquid in the Form of[Choline][NTf₂]

139.82 g (1.0 mol) of choline chloride (2-hyrdoxyethyltrimethylammoniunchloride or vitamin B4; MW 139.82, mp 302 to 305° C.) dissolved in 250ml of distilled water, mixed with 500 ml aqueous solution of 1.0 mol oflithium bis(trifluoromethylsulfonyl)imide ([Li][NTf₂], MW 287.08) in a 1L beaker, stirred at room temperature for 4 hours, and left to stand forphase separation. The upper layer was discarded from the solution, andthe remaining ionic liquid was washed with 200 ml of distilled waterseveral times until no chloride ions (Cl⁻) were detected by silvernitrate test in the washing solution. Upon completion of thepurification, the resulting ionic liquid was placed in a rotaryevaporator and vacuum-dried at 120° C. for more than 6 hours, thusremoving water. As a result, 312.59 g of ionic liquid (C₇H₁₄F₆N₂O₅S₂ MW:384.02) in the form of [Choline][NTf₂] was obtained at a yield of 81.4%.The [choline] was the 2-hydroxyethyltrimethylammonium cation, and the[NTf₂] was the bis(trifluoromethylsulfonyl)imide anion.

The moisture content measured by a Karl Fischer method was 0.493%. NMRresults analyzed by ¹H-NMR (300 MHz, d6-DMSO) were δ=3.099 (3xCH₃), 3.38(O—CH₂—), 3.84 (N—CH₂—), and 5.25 (—OH) and the results analyzed by¹³C-NMR (d6-DMSO) were δ=53.51 (CH₂—O), 55.50 (3xCH₃), 67.42 (N—CH₂—),and 120 (2xCF₃). The results of atomic analysis were C, 21.78%, H,3.71%, N, 7.83%, and S, 17.58%, those were well agreed with thetheoretical atomic ratios of [Choline] [NTf₂] (C₇H₁₄N₂O₅F₆S₂, MW;384.02) are C, 21.88%, H, 3.67%, N, 7.29%, and S, 16.69%. Freezing pointmeasured by differential scanning calorimetry (DSC) was −16° C., meltingpoint was 1.0° C., and specific gravity of liquid at 29.3° C. was 1.520g·cm⁻³.

Example 1 Preparation of DMC at Atmospheric Pressure and 180° C.

A reaction system such as a distillation system equipped with a magneticstirrer capable of stirring the mixture was used. A mantle type heaterwith temperature indicator and controller was used and was equipped witha condenser, an ammonia absorber, and a metering pump. Moreover, thereactor was connected to a metering pump capably of injecting alkylalcohol at a constant flow rate into a reaction solution containing theionic liquid and urea. A preheat coil capable of vaporizing the injectedalkyl alcohol was installed in the reactor such that the alkyl alcoholwas dispersed in the vapor phase to the reaction. When the reactionsystem was established, 100.08 g of ionic liquid [choline][NTf₂]synthesized in the above Synthesis Example and 7.502 g (0.125 mol) ofurea were placed in a 250 ml three-necked round flask reactor and mixedwith 2.003 g of zinc oxide used as a catalyst. Nitrogen was fed into thereactor to replace air, and the temperature of the reactor was increasedto 180±1.0° C. During the reaction, a coolant was circulated to maintainthe temperature of condenser at 5° C. When the reaction temperature wasreached, the supply of nitrogen was cut off, and 60 g (1.873 mol) ofmethanol was fed into the reactor at a flow rate of 0.5 ml/min using ametering pump.

The above process was performed at about 1 atm (i.e., atmosphericpressure), and the sampling of the condensate in the condenser uponcompletion of 2.5 hours of the pure methanol feeding. Then, the samplecollected again from the condensate after every 6.5 and 10.5 hoursduring the condensate was circulated at the same flow rate of 0.5 ml/minusing the metering pump to continue the reaction. The samples wereanalyzed as shown in the following table 1.

When the reaction was continued for 10.5 hours, only MC as an impuritywas present in the condensate. Meanwhile, only N-MC was present in thecondensate and the yield was increased after 14.5 hours, the DMC yieldgradually decreased and some kinds of impurities was appeared. Thedecrease in DMC yield appears to be due to side reaction of DMC to N-MC.

The reactants and products were analyzed by gas chromatography (GC) withan HP-5 capillary column (0.32 mm ID×30 m×1 μm) and an FID detector. Thequantitative analysis of DMC was performed by external standard methodusing heptanol. Moreover, the yield and selectivity were obtained by thefollowing formula 1 and 2, respectively. However, the amounts of dialkylcarbonate and the total product were calculated by analyzing theproducts collected from the condenser.

$\begin{matrix}{{{Math}\mspace{14mu}{Figure}\mspace{14mu} 1}\mspace{515mu}} & \; \\{{{Yield}\mspace{14mu}(\%)} = {\frac{{Dialkyl}\mspace{14mu}{Carbonate}\mspace{14mu}({mol})}{{Urea}\mspace{14mu}({mol})} \times 100\mspace{14mu}(\%)}} & \lbrack {{Math}.\mspace{14mu} 1} \rbrack \\{{{Math}\mspace{14mu}{Figure}\mspace{14mu} 2}\mspace{515mu}} & \; \\{{{Selectivity}\mspace{14mu}(\%)} = {\frac{{Dialkyl}\mspace{14mu}{Carbonate}\mspace{14mu}({mol})}{{{Total}\mspace{14mu}{product}\mspace{14mu}({mol})}\mspace{14mu}} \times 100\mspace{14mu}(\%)}} & \lbrack {{Math}.\mspace{14mu} 2} \rbrack\end{matrix}$

Example 1-1

DMC was prepared in the same manner as in Example 1, except that methylcarbamate was used instead of urea.

Examples 2 to 9 Preparation of DMC at Atmospheric Pressure and 180° C.

DMC was prepared in the same manner as in Example 1, except that CaO,MgO, PbO, and hydrotalcite [Mg₅Al(OH)₁₂(CO₃)_(0.5).4H₂O] and animpregnated catalyst containing 20% by weight of ZnO and 80% by weightof TiO₂ were used as catalysts instead of ZnO catalyst. The yields andthe selectivity according to the reaction times of the prepared DMC wereshown in the following table 1:

TABLE 1 DMC Analysis After 2.5 hours of After 6.5 hours of After 10.5hours of Reaction Reaction Reaction Selectivity Selectivity SelectivityExamples Catalysts Yield (%) (%) Yield (%) (%) Yield (%) (%) Example 1ZnO 23.1 89.4 26.3 94.9 38.8 97.4 Example ZnO 9.3 33.6 21.1 69.0 28.183.5 1-1 Example 2 CaO 17.6 80.6 22.6 85.4 28.3 91.3 Example 3 MgO 26.484.1 30.8 89.8 39.9 94.5 Example 4 PbO 11.1 60.8 37.9 91.6 53.2 95.5Example 5 Y₂O₃ 13.1 61.4 20.3 75.3 29.4 85.4 Example 6 La₂O₃ 6.1 53.77.6 50.6 12.6 58.1 Example 7 Hydrotalcite⁽¹⁾ 34.3 88.6 46.5 95.3 39.485.5 Example 8 Hydrotalcite⁽²⁾ 16.4 82.7 24.0 90.9 25.2 94.2 Example 920 wt % 11.9 68.5 23.3 82.7 20.2 84.4 ZnO80 wt % TiO₂ ⁽¹⁾Hydrotalcite:Mg₅Al(OH)₁₂(CO₃)_(0.6)•4H₂O dried at 120° C. for 12 hours⁽²⁾Hydrotalcite: Mg₅Al(OH)₁₂(CO₃)_(0.6)•4H₂O calcined at 450° C. for 6hours

Examples 10 to 15 & Comparative Examples 1 to 4 Preparation of DMC UsingDifferent Ionic Liquid

DMC was prepared in the same manner as in Example 1, except that thekind of ionic liquids was changed, and the yields and the selectivitiesof the prepared DMCs are shown in the following table 2:

TABLE 2 DMC Analysis After After After 2.5 hours of 6.5 hours of 10.5hours of Reaction Reaction Reaction Yield Selectivity Yield SelectivityYield Selectivity Examples Ionic liquids (%) (%) (%) (%) (%) (%) Example1 [Choline][NTf₂] 23.1 89.4 26.3 94.9 38.8 97.4 Example [HEMin][NTf₂]13.3 88.9 18.2 94.4 21.1 95.9 10 Example [EMin][NTf₂] 9.6 54.5 15.7 76.815.7 80.4 11 Example [HETEA][NTf₂] 16.6 93.1 18.1 96.6 19.6 98.1 12Example [HETBA][NTf₂] 22.1 84.3 27.0 93.4 28.2 94.1 13 Example[TEA][NTf₂] 10.0 55.0 14.1 72.2 16.5 79.6 14 Example [TBA][NTf₂] 11.864.2 15.8 67.9 20.5 75.5 15 Comparative [Betain][NTf₂] 0.3 16.4 0.8 15.30.8 23.2 Example 1 Comparative [Choline][BF₄] 0 0 0 0 0.1 16.4 Example 2Comparative [BMin][BF₄] 1.4 0.2 1.9 0.2 2.6 0.2 Example 3 Comparative[BMin][PF₆] 0.1 0 0.3 0 0.3 0.01 Example 4 [Choline][NTf₂]:hydroxyethyltrimethylammonium bis(trifluoromethylsulfonyl)imide[HEMin][NTf₂]: 1-hydroxyethyl-3-methyl-imidazoliumbis(trifluoromethylsulfonyl)imide [EMin][NTf₂]:1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide[HETEA][NTf₂]: hydroxyethyltriethylammoniumbis(trifluoromethylsulfonyl)imide [HETBA][NTf₂]:hydroxyethyltributylammonium bis(trifluoromethylsulfonyl)imide[TEA][NTf₂]: tetraethylammonium bis(trifluoromethylsulfonyl)imide[TBA][NTf₂]: tetrabutylammonium bis(trifluoromethylsulfonyl)imide[Betain][NTf₂]: 1-carboxy-N,N,N-trimethylmethanaminium hydroxidebis(trifluoromethylsulfonyl)imide [Choline][BF₄]:hydroxyethyltrimethylammonium tetrafluoroborate [BMin][BF₄]:1-butyl-3-methyl-imidazolium tetrafluoroborate [BMin][PF₆]:1-butyl-3-methyl-imidazolium hexafluorophosphate

[Betain][NTf₂] in Comparative Example 1 is an ionic liquid having acarboxyl cation which produces a hydrogen ion (H⁺) but dissolves themetal oxide catalyst to deactivate the active sites of a solid catalyst.As a result, it was shown that the [Betain][NTf₂] exhibited low yieldand low selectivity of DMC. The ionic liquids having a tetrafluoroborate(BF₄) or hexafluorophosphate (PF₆) anion in Comparative Examples 2 to 4exhibited very low reactivities of DMC. Moreover, the tetrafluoroborateand hexafluorophosphate were reacted with methanol as a reactant toproduce by-products such as trimethoxyboroxane and methoxy phosphoruscompounds, which reduces the selectivity to DMC and generateshydrofluoric acid (HF) to corrode the reactor.

However, it was confirmed that the DMC prepared in Examples 1, and 9 to14 using the ionic liquids provided by the present invention had highyields and high selectivities.

Comparative Examples 5 to 8 Preparation of DMC Using PGDE Solvent

DMC was prepared in the same manner as in Example 1, except that 100 gof polyethylene glycol dimethyl ether (PGDE, MW 250 to 270) as a highboiling electron donor organic compound was used instead of the ionicliquid and different catalysts were used. The yields and selectivitiesof the thus prepared DMC are shown in the following table 3:

TABLE 3 DMC Analysis After After After 2.5 hours of 6.5 hours of 10.5hours of Reaction Reaction Reaction Comparative Yield Selectivity YieldSelectivity Yield Selectivity Examples Catalysts (%) (%) (%) (%) (%) (%)Comparative ZnO 8.4 0.02 12.4 0.02 12.2 0.02 Example 5 Comparative TiO₂0.11 0.00 0.11 0.00 0.16 0.00 Example 6 Comparative C₆₆H₇₀O₄Zn* 0.0 0.000.38 0.00 0.72 0.00 Example 7 Comparative C₆₆H₇₀O₄Zn 1.03 0.00 1.58 0.001.66 0.00 Example 8 *reaction temperature (150° C.)

It can be seen from Table 3 that the yields with respect to the amountof urea added were all low except for those in Comparative Example 5using the zinc oxide catalyst and the selectivities were very bad due toan increase in impurities. The reason for this is considered that thehigh boiling solvent PGDE was decomposed at the reaction temperature of180° C. and distilled with a significant amount of low molecularby-products together with a condensate, thus reducing the selectivity,which considerably increases the cost of purification thus having noeconomical advantage.

Examples 16 and 17 Preparation of Diethyl Carbonate and di-n-propylCarbonate at Atmospheric Pressure and 180° C.

Diethyl carbonate and di-n-propyl carbonate were prepared in the samemanner as in Example 1, except that ethyl alcohol and 1-propyl alcoholwere used instead of methanol and a solution having a molar ratio ofethanol (or n-propanol) to urea of 10:1 was prepared and injected at aconstant flow rate of 0.5 ml/min. The results obtained are shown in thefollowing table 4:

TABLE 4 DMC Analysis After After After 2.5 hours 6.5 hours 10.5 hours ofReaction of Reaction of Reaction Selec- Selec- Selec- Dialkyl Yieldtivity Yield tivity Yield tivity Examples carbonates (%) (%) (%) (%) (%)(%) Example diethyl 10.9 94.3 15.5 96.0 17.6 98.3 16 carbonate Exampledi-n- 8.8 67.6 12.4 79.8 13.0 95.8 17 propyl carbonate

It can be seen from Table 4 that it was possible to obtain good yieldsof more than 13% and dialkyl carbonate of high purity containing noimpurities other than the intermediate products such as ethyl carbamateand n-propyl carbamate even in the reactions using ethyl alcohol and1-propyl alcohol.

Examples 18 to 20 Preparation of DMC at Different Temperatures

DMC was prepared in the same manner as in Example 4, except that a metaloxide catalyst (PbO) was used at different temperatures and the analysisresults are shown in the following table 5:

TABLE 5 DMC Analysis After 10.5 hours of After After Reaction 2.5 hoursof 6.5 hours of Se- Reaction Reaction lec- Temp. Yield Selectivity YieldSelectivity Yield tivity Examples (° C.) (%) (%) (%) (%) (%) (%) Example160 3.0 50.4 3.7 50.6 5.4 53.0 18 Example 4 180 11.1 60.8 37.9 91.6 53.295.5 Example 200 37.4 85.6 38.6 94.4 32.0 95.6 19 Example 220 27.1 80.912.5 89.3 6.7 91.8 20

When the reaction temperature was 160° C., the yield and selectivity ofDMC were low due to the MC synthesized by the first step reaction.However, since no other impurities were produced, it was possible toincrease the yield of DMC by recirculating MC and methanol remaining inthe reboiler after azeotropic distillation of DMC and methanol.Meanwhile, when the reaction temperature was 220° C., the N-methylcarbamate was produced as a by-product by the reaction of DMC and MCafter 2.5 hours of the reaction, and thus the selectivity was graduallydecreased. Even in this case, it is possible to increase theproductivity by an increase in the reaction rate if the MC and methanolremaining in the reboiler are recirculated after collecting the producedDMC by azeotropic distillation.

Examples 21 to 24 Preparation of DMC in Different Amounts of Catalyst atAtmospheric Pressure and 180° C.

DMC was prepared in the same manner as in Example 3, except that theamount of MgO catalyst used was changed as shown in the following table6:

TABLE 6 DMC Analysis After After After 2.5 hours of 6.5 hours of 10.5hours of Amount Reaction Reaction Reaction Ionic Of Yield SelectivityYield Selectivity Yield Selectivity Examples liquid catalyst (%) (%) (%)(%) (%) (%) Example [choline][NTf₂] 0.5 21.5 79.8 26.8 87.6 28.6 91.6 21Example 1.0 23.1 82.8 29.5 91.4 31.8 94.3 22 Example 3 2.0 26.4 84.130.8 89.6 39.9 94.5 Example 3.0 23.4 83.7 31.6 92.8 30.5 94.7 23 Example4.0 26.9 87.4 32.7 94.7 33.5 96.6 24

Examples 25 to 29 Preparation of DMC in Different Amounts of Urea UnderAtmospheric Pressure and at 180° C.

DMC was prepared in the same manner as in Example 1, except that theamount of urea used was changed and a ZnO catalyst dried at 120° C. for12 hours was used. The yields and selectivities of DMC are shown in thefollowing table 7:

TABLE 7 DMC Analysis Molar After After After ratio of 2.5 hours of 6.5hours of 10.5 hours Amount methanol Reaction Reaction of Reaction of toYield Selectivity Yield Selectivity Yield Selectivity Examples urea urea(%) (%) (%) (%) (%) (%) Example 5.623 20.0 20.3 90.0 19.6 95.6 20.3 93.425 Example 6.426 17.5 17.2 87.4 24.9 95.0 23.8 95.2 26 Example 1 7.50015.0 23.1 89.4 26.3 94.9 38.8 97.4 Example 8.996 12.5 14.9 88.2 22.094.3 21.9 93.6 27 Example 11.245 10.0 16.7 85.0 16.9 87.5 24.4 92.8 28Example 14.994 7.5 10.0 60.5 16.2 74.5 17.5 78.2 29

The above results were obtained by varying the molar ratio of methanolto urea relative to 100 g of the ionic liquid. When the amount of ureaused was in the range of 5 to 15 g, there was no significant change inthe yield. When the amount of urea used was large, the concentration ofMC as an intermediate product was increased, thereby decreasing theselectivities. Meanwhile, when the amount of urea used was small, therewas no significant change in the yield and selectivity, but theproductivity was reduced, thus not being economical.

Examples 30 to 34 Preparation of DMC at Different Flow Rates of Methanoland Urea Fed at the Same Time

DMC was prepared in the same manner as in Example 1, except that asolution prepared by dissolving 7.5 g of urea in 60 g of methanol (molarratio of methanol to urea was 15:1) was fed into the reactor atdifferent flow rates as shown in the following table 8:

TABLE 8 DMC Analysis After After After 2.5 hours of 6.5 hours of 10.5hours Flow Reaction Reaction of Reaction rate Reaction Yield SelectivityYield Selectivity Yield Selectivity Examples (ml/min) temp. (%) (%) (%)(%) (%) (%) Example 0.25 180° C. 0.5 100 8.8 96.8 27.1 94.5 30 Example0.5 5.7 85.1 16.3 80.0 18.1 86.0 31 Example 1.0 15.0 68.1 18.0 71.7 21.249.9 32 Example 2.0 10.0 61.2 12.3 48.5 12.7 49.9 33 Example 3.0 10.638.9 13.4 46.2 14.2 42.7 34

As shown in Examples 30 to 34, the yield and selectivity of DMC wererelated to the flow rate. At high flow rate of methanol was shownclosely related to factors such as the type of the disperser of methanolvapor in the reactor, and contact time of reactants in the ionic liquid.Therefore, the decrease in the yield and selectivity is ascribed to thefollowing reasons. That is, the gaseous methanol of the reactant was notsmoothly dispersed and its contact time with the reactants was notsufficient due to the high flow rate, thus reducing the yield of DMC,and the selectivity was reduced due to MC synthesized by the first stepof the reaction.

However, it is possible to increase the yield of DMC by redistilling theproduced condensate to obtain an azeotropic mixture of 30% by weight ofDMC and 70% by weight of MeOH at 62.7° C. and re-circulate the methanoland MC remaining in the reboiler.

In the present invention, when the flow rate is low, the yield by usingthe ionic liquid slurry catalyst system is increased but theproductivity is reduced. On the contrary, when the flow rate isincreased, the yield is reduced but the productivity is increased if themethanol and MC are re-circulated by the distillation system employed.However, a significant increase or decrease in the flow rate reduces theyield and the productivity, which is undesirable. Therefore, in thepresent invention, alkyl alcohol may be fed at a flow rate of 0.1 to 10ml/min, preferably at a flow rate of 0.1 to 5 ml/min, more preferably0.1 to 3 ml/min; however, the present invention is not limited thereto.

The conventional methods for preparing dialkyl carbonates have theproblems that the amount of energy required for the separation andpurification of the products is increased owing to the amount ofby-products produced at high temperature and high pressure. Moreover,since the concentration of the produced DMC is lower than that atatmospheric pressure due to a change in the boiling point at highpressure when the synthesis is performed by the reactive distillation,the productivity is reduced and the energy cost is increased due to therecirculation of methanol as a raw material. Further, when the reactionpressure is high, the cost of equipment for safety is increased, and thecost of a pressure control device required to control the pressure isalso increased. As such, there are many problems that the reaction yieldor selectivity is low due to the by-products such as N-MC and NN-DMC,the cost of equipment is increased, and a large amount of energy isrequired due to the complicated process and high pressure.

However, as can be confirmed from the above-described Examples, thepresent invention can prepare dialkyl carbonate at atmospheric pressurewith high yield and selectivity. Therefore, since the present inventiondoes not require expensive pressure control device and peripheraldevices for maintaining high pressure, it is possible to reduce theinstallation cost and prepare dialkyl carbonate with high yield, thusimproving the cost effectiveness.

Moreover, since the used ionic liquid can be easily recycled bypurification and there is hardly no waste produced thereof, the methodfor preparing dialkyl carbonate of the present invention is anenvironment-friendly method.

The invention has been described in detail with reference to preferredembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

The invention claimed is:
 1. A method for preparing dialkyl carbonate byreacting alkyl alcohol with urea or alkyl carbamate in the presence of:an ionic liquid comprising a cation capable of generating a hydrogen ion(H+), and a hydrophobic anion containing fluorine, and a catalyst,wherein said cation is a quaternary ammonium cation, an imidazoliumcation, a pyridium cation, a pyrazolium cation, a pyrrolinium cation, aquaternary phosphonium cation, a thiazolium cation, or a sulfoniumcation, and said hydrophobic anion is bis(trifluoromethylsulfonyl)imide,trifluoromethanesulfonate, or tris(trifluoromethylsulfonyl)methanide. 2.The method for preparing dialkyl carbonate according to claim 1, whereinthe quaternary ammonium cation contains at least one substituentselected from the group consisting of a C₁-C₅ hydroxyalkyl group, aC₁-C₅ alkoxy group, and a C₁-C₅ alkyl group.
 3. The method for preparingdialkyl carbonate according to claim 2, wherein the quaternary ammoniumcation is a hydroxymethyltrimethylammonium cation, ahydroxyethyltrimethylammonium cation, a hydroxyethyltriethylammoniumcation, a hydroxyethyltripropylammonium cation, ahydroxyethyltributylammonium cation, a tetraethylammonium cation, or atetrabutylammonium cation.
 4. The method for preparing dialkyl carbonateaccording to claim 3, wherein the imidazolium cation is a1,3-di(C₁-C₅)alkyl-imidazolium cation or a1-hydroxy(C₁-C₅)alkyl-3-(C₁-C₅)alkyl imidazolium cation.
 5. The methodfor preparing dialkyl carbonate according to claim 1, wherein thehydrophobic anion is bis(trifluoromethylsulfonyl)imide.
 6. The methodfor preparing dialkyl carbonate according to claim 1, wherein the ionicliquid is [ethyltrimethylammonium][bis(trifluoromethylsulfonyl)imide],[hydroxyethyltriethylammonium][bis(trifluoromethylsulfonyl)imide],[1-hydroxyethyl-3-methyl-imidazolium][bis(trifluoromethylsulfonyl)imide],or [1-ethyl-3-methylimidazolium][bis(trifluoromethylsulfonyl)imide]. 7.The method for preparing dialkyl carbonate according to claim 1, whereinthe catalyst comprises at least one selected from the group consistingof CaO, MgO, ZnO, PbO, La₂O₃, Y₂O₃ and hydrotalcite.
 8. The method forpreparing dialkyl carbonate according to claim 1, wherein the catalystis used in an amount of 1 to 10 parts by weight per 100 parts by weightof the ionic liquid.
 9. The method for preparing dialkyl carbonateaccording to claim 1, wherein the urea or alkyl carbamate is used in anamount of 1 to 30 parts by weight per 100 parts by weight of the ionicliquid and the alkyl alcohol is used in a molar ratio of 5 to 25 permole of the urea or alkyl carbamate.
 10. The method for preparingdialkyl carbonate according to claim 1, wherein the reaction is carriedout in a temperature range from 140 to 240° C.
 11. The method forpreparing dialkyl carbonate according to claim 10, wherein the reactionpressure is carried out below a saturated vapor pressure of alkylalcohol or atmospheric pressure at reaching the reaction temperature.12. The method for preparing dialkyl carbonate according to claim 10,wherein the reaction is carried out at or below atmospheric pressure.13. A method for preparing dialkyl carbonate by reacting alkyl alcoholwith urea or alkyl carbamate in the presence of: an ionic liquidcomprising a cation capable of generating a hydrogen ion (H+), and ahydrophobic anion containing fluorine, and a catalyst, wherein saidcation is a hydroxymethyltrimethylammonium cation, ahydroxyethyltrimethylammonium cation, a hydroxyethyltriethylammoniumcation, a hydroxyethyltripropylammonium cation, ahydroxyethyltributylammonium cation, a tetraethylammonium cation, or atetrabutylammonium cation, and said hydrophobic anion isbis(trifluoromethylsulfonyl)imide.
 14. The method for preparing dialkylcarbonate according to claim 13, wherein the ionic liquid is[ethyltrimethylammonium][bis(trifluoromethylsulfonyl)imide],[hydroxyethyltriethylammonium][bis(trifluoromethylsulfonyl)imide],[1-hydroxyethyl-3-methyl-imidazolium][bis(trifluoromethylsulfonyl)imide],or [1-ethyl-3-methylimidazolium][bis(trifluoromethylsulfonyl)imide]. 15.The method for preparing dialkyl carbonate according to claim 13,wherein the catalyst comprises at least one selected from the groupconsisting of CaO, MgO, ZnO, PbO, La₂O₃, Y₂O₃ and hydrotalcite.
 16. Themethod for preparing dialkyl carbonate according to claim 13, whereinthe catalyst is used in an amount of 1 to 10 parts by weight per 100parts by weight of the ionic liquid.
 17. The method for preparingdialkyl carbonate according to claim 13, wherein the urea or alkylcarbamate is used in an amount of 1 to 30 parts by weight per 100 partsby weight of the ionic liquid and the alkyl alcohol is used in a molarratio of 5 to 25 per mole of the urea or alkyl carbamate.
 18. The methodfor preparing dialkyl carbonate according to claim 13, wherein thereaction is carried out in a temperature range from 140 to 240° C. 19.The method for preparing dialkyl carbonate according to claim 18,wherein the reaction pressure is carried out below a saturated vaporpressure of alkyl alcohol or atmospheric pressure at reaching thereaction temperature.
 20. The method for preparing dialkyl carbonateaccording to claim 18, wherein the reaction is carried out at or belowatmospheric pressure.